1. Technical Field
The invention relates to a liquid crystal display device, and more particularly, to a backlight driving system for a liquid crystal display device.
2. Related Art
Generally, display devices are compact and lightweight. Although Cathode Ray Tubes (“CRT”) have been widely used for television monitors, a measuring system and an information terminal, they do not provide a compact and light display device due to their inherent size and weight. Accordingly, CRTs have been replaced by other display devices such as a liquid crystal display (“LCD”) device, a plasma display panel (“PDP”) and an electroluminescence display (“ELD”) device. Among those display devices, LCD devices use an electric field optical effect and can provide advantages such as low power consumption and a slim, lightweight structure. As a result, applications of LCD devices range from monitors for personal computers, including desktop and laptop computers, to large size display devices.
Some LCD devices control light transmittance from ambient light to display images. Others use an additional light source, such as a backlight unit, in an LCD panel. FIG. 1 illustrates a circuit diagram of a backlight driving system 1 for a LCD device. Referring to FIG. 1, a backlight is a lamp 10 that emits light to a liquid crystal display panel (not shown). The lamp 10 may be a cold cathode fluorescent lamp (CCFL). The backlight driving system includes a first inverter 11, a second inverter 12, a first transformer 13 and a second transformer 14. The first inverter 11 outputs a driving voltage to a first terminal 2 of the lamp 10 in accordance with a control signal of a timing controller 15. Likewise, a second inverter 12 outputs a driving voltage to a second terminal 4 of the lamp 10 in accordance with a control signal of the timing controller 15. Then, the first transformer 13 transforms an output voltage of the first inverter 11 and supplies a transformed output to the first terminal 2 of the lamp 10. In the same manner, the second transformer 14 transforms an output voltage of the second inverter 12 and supplies a transformed output to the second terminal 4 of the lamp 10. Each input coil 5, 5′ of the first and the second transformers 13 and 14 is connected to output terminals 6, 8, 6′, 8′ of the first and second inverters 11 and 12, respectively. Each output coil 7, 7′ of the first and the second transformers 13 and 14 are connected to the first terminal 2 and the second terminal 4 of the lamp 10.
The first inverter 11 includes a first transistor, a second transistor, a third transistor, and a fourth transistor M1, M2, M3, and M4. The third transistor M3 and the first transistor M1 are connected in series between a voltage terminal (VCC) and a ground terminal (GND). The fourth transistor M4 and the second transistor M2 are connected in series between the voltage terminal (VCC) and the ground terminal (GND). The first output terminal 6 is formed between the third transistor M3 and the first transistor M1, and the second output terminal 8 is formed between the fourth transistor M4 and the second transistor M2. Thus, the first and second output terminals 6, 8 are each connected to the input coil 5 of the first transformer 13.
The second inverter 12 has the same structure as the first inverter 11 as described above. Specifically, the first output terminal 6′ is formed between the third transistor M3 and the first transistor M1, and the second output terminal 8′ is formed between the fourth transistor M4 and the second transistor M2. Thus, the first and second output terminals 6′, 8′ are each connected to the input coil 5′ of the second transformer 14.
A dot (●) marked on the input coils 5, 5′ of the transformers 13, 14 indicates a starting point of the input coil 5, 5′. Volts Alternating Current (“VAC”) is a sine wave that is outputted from the first and the second transformers 13, 14. A VAC outputted from the second transformer 14 has an inverted phase from a VAC outputted from the first transformer 13.
The backlight driving system 1 described above has the following disadvantages. The system 1 requires the first inverter 11, the second inverter 12, the first transformer 13 and a second transformer 14 to supply a desired voltage to the first and the second terminals 2, 4 of the lamp 10. Accordingly, the system 1 is large in size and the power consumption increases. Also, fabrication cost substantially increases. In addition, due to a difference in impedance generated between each load of the first inverter 11⇄first transformer 13⇄lamp 10 and the second inverter 12⇄second transformer 14⇄lamp 10, non-uniform voltage may be transmitted to each end terminal 2, 4 of the lamp 10. This non-uniform voltage reduces product reliability.
Use of only one inverter and one transformer may not provide the desired uniformity or equally divide and output the voltage. This, a single inverter/transformer backlight driving system provides non-uniform and unequal voltages that may be transmitted to each end terminal of a lamp. This non-uniform and unequal voltage results in non-uniform brightness of the lamp.